The worst thing that can happen to writers is that nobody reads their work. I used to get upset when I found some of my books were being pirated, but then I realised that I was getting free publicity. Many readers, when they finish a book they have enjoyed, pass it to friends or family members. The book often ends up in a second hand or charity shop. Authors have no choice but to accept this, but what is unacceptable is that there are unscrupulous individuals out there who exploit the endeavours of an author for their own gain.
Sometimes a reader will not like a book and may leave an unfavourable or even a bad review. It happens as a writer cannot please everyone and hopefully those occasions will be very few. But at least this person will have read the book. Of course, if it happens too much then the author must seriously rethink how they go about their trade. A new author or even an establish author will give away free copies of a new book in exchange for a review. It is very unusual for a reviewer to take advantage of this and sell the book on, but it can happen.
I have been surprised to find some of my books for sale online for far more money than they are advertised by my own website or reputable book stores. There is a market for early editions, especially those signed by the author. Some early copies of my first books, complete with typing and formatting errors, have even ended up online for exorbitant amounts. There is nothing an author can do about this. But it is publicity and how much income authors can loose must be offset against the free publicity they get.
Most authors earn little from selling their work. I know very few who earn a living from their writing full time. They write because they enjoy their craft, but above all they love to bring enjoyment to readers. The best thing that can happen to a writer is when a reader who you have never met picks up your book and writes to you to say how much they have enjoyed it. Those are the moments I savour.